1. Field of the Art
The invention relates to neutron absorbing bars for liquid cooled nuclear reactors of the type having a cluster of vertical parallel neutron absorbing rods fixed to arms of a spider having a central pommel connectable to a vertical moving mechanism and a damping device in the pommel. It is particularly suitable for use in pressurized water cooled and moderated reactors.
2. Prior Art
Neutron absorbing bars for nuclear reactors include rods which contain a neutron poison for controlling the reactivity in the core of the reactor. They are inserted into the core to a variable degree.
To cause an emergency shutdown of the reactor, all control bars are simultaneously lowered into the core by dropping them so that they enter the core under the action of their own weight.
To damp the shock when the pommel abuts against the upper core plate of the reactor or against the upper end piece of a respective fuel assembly, the provision of a shock damper has already been proposed. A control bar described in European Patent No. 159 509 has a damping device consisting of a cylinder formed in the pommel and slidably receiving a piston urged downwardly by resilient means contained in the cylinder. From the moment when the piston abuts the upper core plate, continued downward movement is opposed by the compression of the resilient means and by the pressure loss undergone by the liquid which flows out of the cylinder between the wall thereof and the piston. Such a damping device has, however, only a limited effect: the damping effect due to the pressure loss does not change substantially during movement of the piston and only the increasing stiffness of the spring provides progresivity. Furthermore, the device in the pommel interferes with the flow of cooling liquid.
Such shortcomings could be accepted for bars whose rods contain a neutron poison in coherent form and which do not require substantial cooling. It is no longer acceptable when the bars contain other compounds of limited resistance, and particularly when the rods of the bar contain a material used for varying the energy spectrum of the neutrons in the core. This material often consists of fertile material pellets (depleted uranium oxide, and/or thorium oxide for example) which do not withstand shocks.